Painted newly plastered walls - yellow stains/patches

[Carlos8881]

Recently had new room plastered. been left to to dry for 2-3 weeks - plaster pink and ready to be painted.

Did mist coat this weekend - and overnight noticed these random yellow/dull stains in random parts of the walls/celin. its in about 3 or 4 places overall in small patches.

The photos dont show its yellowness very well - but what could/would this be?
there is also no pipes nearby as suspected water issue but nope.

Any insight into what it may be - and maybe how to resolve? i keep reading about Stainblock - so likely head down this route but ideally like to know whats causing issue.

thanks

 

[Lee dorey]

Looks like damp stains but obviously any entrapped water causes new plaster to dry differently which causes colour differences really if the marks were there before and it’s been reskimmed they probably just used pva instead of grit

 

[Lee dorey]

Also if smoked in at any time nicotine might cause it

 

[Wayners]

Loft above?
Maybe no rockwool insulation along the edge and it's a cold spot. Condensation forums and mould then it stains

 

[york plasterers]

It could well be they were not fully dry also like other answers something behind them, you could use a blocker but that's just masking the issue. Just realised you said a new room is this an extension so first time plastered if so is the rooom now heated and been lived in ? or an existing room in house? If it's new could well be damp from build still in timbers. If its an existing room refurb then it could well be that the damp is present through an old gap that has been allowing moisture/water in over time.If that s the case need to find the external gap. Failing all else and you just are happy to stop the discoloration try Zinsser cover stain get the oil based not water based also quick video on this site painting new plaster here

 

[opps]

What do you mean when you say that it is a new room?

Old water stains can bleed through to re-skims.

 

[JohnD]

Yellow stain is often water from a leak. In the position you show, it might be roof or gutter related. It may be an old leak that was later repaired.

What is above?

Please photograph from outside, and also the room or loft above